Immigration reform, if not now, when?

From: http://news.yahoo.com/s/ap/20100429/ap_on_go_pr_wh/us_immigration_politics

“The president noted that lawmakers may lack the “appetite” to take on immigration while many of them are up for re-election”

Perhaps I am a little naive, BUT.

I thought that every two years, there are the same number of lawmakers up for re-election.

1/3 of the senate and all of the house.

This never changes, so when do they actually decide to take on immigration?